Add Support for Additional Model Classes

[leeper]

Full support

• "betareg" from betareg
• "ivreg" from AER
• "glm" from stats
• "lm" from stats
• "merMod" from lme4
• "svyglm" from survey

Provisional support (may not be fully tested)

• "clm" from ordinal
• "nnet" from nnet
• "polr" from MASS
• "ordinal" from ordinal

Support in progress

• "multinom" from nnet
  • Problem: coef() is a matrix and vcov() has weird names

Requested

Supported in prediction (see leeper/prediction#1):

• "gee" from gee
• "glmnet" from glmnet

Not yet supported in prediction:

• "ivprobit" from ivprobit
• geepack
• lfe
• gls
• systemfit
• glmmTMB

[mikenguyen13]

Hi,
I was wondering if you can also support the "fixest" object from fixest. Thank you so much.